It is known that a stable aqueous resin dispersion can be obtained by carrying out normal emulsion polymerization using a vinyl monomer containing (meth) acrolein, and processes which are disclosed in U.S. Pat. No. 4,988,762, Kobunshi Kagaku 1971, Vol. 28, No. 313 PP. 430 through 436, etc. are known. In the emulsion polymerization, since polymer conversion of the (meth) acrolein is not 100%, after a polymerization reaction is completed, it is necessary to distill off unreacted (meth) acrolein from an aqueous resin dispersion. However, with the conventional emulsion polymerization method, it is hard to distill off the unreacted (meth) acrolein from the aqueous resin dispersion. Therefore, aqueous resin dispersion obtained by the conventional emulsion polymerization method has a problem of a pungent smell and toxicological properties which are peculiar to (meth) acrolein.
Further, in the case where a copolymer containing a lot of (meth) acrolein is prepared by the conventional emulsion polymerization method, a lot of oligomers whose number-average molecular weight (Mn) is below 1000 originated in a homopolymer of (meth) acrolein is produced. These oligomers have toxicological properties, so the aqueous resin dispersion has toxicological properties originated in the oligomer.
For this reason, the aqueous resin dispersion obtained by the conventional emulsion polymerization method has a problem of a difficulty in practical use.
On the other hand, a polyvinyl alcohol is peculiar synthetic resin which dissolves in water, and has a strong adhesive force and emulsification force, and forms a strong film. For these reasons, the polyvinyl alcohol is widely applied to various uses. The applications are, for example, vinylon fiber, film, adhesive, fabric size, fabric processing agent, paper agent, emulsification dispersant, etc. Moreover, emulsified material in which the polyvinyl alcohol is used as protective colloid, especially, polyvinyl acetate resin emulsion is used as coating material, adhesive, mortar binder, etc.
Further, since starch is a kind of a polysaccharide and has a strong adhesive force and forms a strong film. For these reasons, starch is widely applied to various uses. The applications are, for example, yarn-making size, fabric processing agent, filler, adhesive, etc.
However, since a polyvinyl alcohol group and starch are water-soluble, moldings containing mainly the polyvinyl alcohol and starch, and products using partly the polyvinyl alcohol and starch are inferior in water resistance, hot water-resistance, etc. In the cases especially where polyvinyl acetate resin emulsion is used as painting material, adhesive, mortar binder for moldings, etc., and where starch is used as filler for a wood material fibreboard and mineral material fibreboard, since these products are mostly used in the fields of public civil engineering and construction industry which are done outdoors, the problem of the water resistance becomes serious. Therefore, a water-resistant improver which is capable of improving water resistance and hot water resistance of products and compositions containing a polyvinyl alcohol and starch is desired as a kind of a water-resistant improver.
Here, in order to improve water-resistance and hot water resistance, etc. of products and compositions a containing polyvinyl alcohol and starch, various water-resistant improvers have been examined. However, for example, in the case where low-molecular aldehyde compound such as glyoxal is used as a water-resistant improver, a pot life is short and enough hot water-resistance is not provided. Moreover, in the case where urea resin, melamine resin, phenol resin, etc. are used as a water-resistant improver, there arises problems that heating treatment at 100.degree. C. and above is required and free formalin, etc. is generated. Further, a water-resistance improving method by using aqueous vinyl urethane obtained by dispersing isocyanate compound in water with hydrophobic organic solvent (Japanese Examined Patent Publication No. 30576/1976 (Tokukosho 51-30576)) is reported. However, in this case, problems that a pot life is extremely short in the use under condition of more than 30.degree. C. are left and toxicological properties are high. Moreover, a process for adding metallic salt such as aluminum chloride, aluminum nitrate, chrome nitrate, zinc chloride, zirconium nitrate, as a water-resistant improver is proposed. However, in the case where products and compositions where the water-resistant improver is used is disposed, since metal component originated in the metallic salt flows out, such products and compositions have an disadvantage which causes environmental pollution. As mentioned above, the conventional water-resistant improver is not practical because of its various problems.
In addition, a paper strength additive which reinforces wet strength of paper is known as a kind of a water-resistant improver. Polyamide-polyamine-epichlorohydrin resin is widely used as the paper strength additive. However, the conventional paper strength additive can provide strong wet strength, but its strong wet strength deteriorates breakability of paper. For this reason, when the conventional paper strength additive is used, there arises a problem that broken paper is difficult to be recovered (recycle of paper). Here, a paper strength additive which provides excellent breakability is also known, but this paper strength additive has weaker wet strength than the polyamide-polyamine-epichlorohydrin resin. Therefore, a paper strength additive, which can simultaneously provide wet strength equivalent to the polyamide-polyamine-epichlorohydrin resin and excellent breakability, is desired.
In addition, it is known that a (meth) acrolein monomer shows antifungal properties, and it is disclosed in, for example, Kirk-Othmer. "Encyclopedia of Chemical Technology" (3rd. ED.), Vol. 1, Wiley-Interscience (New York), etc. Moreover, a (meth) acrolein copolymer is disclosed as an anti-fungus agent in U.S. Pat. No. 5,290,894. However, the above publication does not disclose any process for a (meth) acrolein copolymer which is used as an aqueous resin dispersion, and any problems of a pungent smell, toxicological properties, etc. due to unreacted (meth) acrolein in the aqueous resin dispersion, an oligomer, etc. to be by-produced. Furthermore, in order to provide effective antifungal properties to various products and compositions, it is necessary to increase a content of (meth) acrolein unit in a copolymer. Therefore, an anti-fungus agent which can provide effective antifungal properties to various products and compositions is desired.